The use of optical fiber communication systems has increased significantly during the last few years. It appears likely that the use of this mode of communications will continue to increase in the future. Not surprisingly, companies engaged in the manufacture of components for these systems continue to seek ways to reduce their costs. One approach is to enhance the efficiencies of handling materials involved in the production of optical preforms, from which optical fibers may be drawn.
Presently, optical preforms are being manufactured in a number of different processes which include vapor deposition as a materials-forming technique. These processes are used to manufacture optical preforms which is a very early step in making lightguide optical fibers. One such process which is known as a modified chemical vapor deposition (herein after MCVD) process is described in J. B. MacChesney "Materials and Processes for Preform Fabrications--Modified Chemical Deposition," Vol. 64, proceedings of IEEE, pages 1181-1184 (1980).
Input to the MCVD process generally comprises a carrier gas and reactant vapors such as germanium tetrachloride (GeCl.sub.4), silicon tetrachloride (SiCl.sub.4) and phosphorous oxychloride (POCl.sub.3). These reactant vapors are supplied from vaporizers commonly referred to as deposition bubblers and are passed to a deposition site such as a glass substrate tube. An optical preform is manufactured by sequentially heating portions of the substrate tube to a temperature in the range of 1600.degree. C. to 1800.degree. C. to react the vapors as they flow through the bore of the tube and deposit them within the substrate tube. In the manufacture of preforms using the MCVD technique, the reactant vapors need to be mixed or blended precisely and delivered at controlled concentration levels to the substrate tube (as opposed to a torch for other manufacturing techniques discussed later). To date, such controlled delivery has been achieved by bubbling a carrier gas such as oxygen (O.sub.2), argon (Ar), helium (He) and/or nitrogen (N.sub.2), for example, through heated supplies of the reactant materials in liquid form in bubblers and then to the deposition site with the vapors entrained in the carrier gases.
Typically, a deposition bubbler includes a container in which a carrier gas intake conduit terminates in an orifice located below the free surface of liquid contained therein. An outlet conduit provides fluid communication between the space above the surface of the liquid and the vapor deposition site. Exemplary of deposition systems employing bubblers is that illustrated in U.S. Pat. Nos. 3,826,560, and 4,276,243.
Inasmuch as vapor of the liquid contained within a deposition bubbler is withdrawn during deposition, the level of liquid drops unless the bubbler is replenished from an auxiliary source. In some applications, decreases in the level of liquid within the bubbler have little effect. In other applications, however, such as in vapor deposition processes employed in the manufacture of optical fiber preforms, variations in the liquid level may have an adverse effect such as changing the concentration level of the delivered vapor.
The vaporization rate also is dependent upon several other factors including the flow characteristics of the carrier gas bubbled through the liquid. For example, the size of the bubbles, as they rise through the liquid, has an effect on the rate of vaporization. The rate of flow of the carrier gas introduced into the bubbler also affects the rate of vaporization, as does the residence time of the bubbles which, of course, depends on the depth at which the carrier gas is introduced and in turn, on the rate of replenishing the liquid relative to the rate of use as discussed above. Another factor is the control of the heat transfer into the bubbler which is affected by significant changes in the quantity of liquid in the bubbler. Although it is possible to program a heater controller to account for some of these variables as changes in the level of liquid are continuously monitored, that approach is complex and does not satisfy completely the needed control for vapor delivery.
In addition to the references listed above, U.S. Pat. No. 4,235,829 which issued on Nov. 25, 1980 to Fred P. Partus is noted. In it, there is shown a vapor delivery system which comprises a deposition bubbler adapted to generate and to deliver vapor from a liquid contained therein and in a reservoir in fluid communication with the bubbler. Facilities are provided for sensing the level of the liquid contained within the bubbler and for providing gaseous head pressures in the reservoir of magnitudes dependent upon the sensed liquid level. The liquid level in the bubbler drops as liquid is vaporized and withdrawn from the bubbler whereupon the level is adjusted by increasing the pressure head in the reservoir to feed liquid to the bubbler. Although the system works well, perturbations in the deposition bubbler caused by a drop in liquid level and then a rise due to the changing of the pressure in the reservoir can to an extent affect adversely the rate of vaporization and hence the concentration level of the vapor. These level changes are increased as the rate of deposition and hence the rate of withdrawal of vapor are increased.
Additionally, commonly-assigned U.S. Pat. No. 4,276,243 which issued on Jun. 30, 1981 to Fred P. Partus discloses a similar vapor delivery system which utilizes the temperature of the liquid as the characteristic to be monitored and manipulated to control the concentration level at the desired value. However, while this system is effective, the slow rate at which the overall temperature of the liquid can be changed, particularly cooled, often results in an unwanted delay in achieving the desired concentration level corrections.
There also exists systems in which, for example, one bubbler is positioned within another with both bubblers being depleted substantially sequentially. An exemplary system of this type is described in U.S. Pat. No. 4,582,480, which issued on Apr. 15, 1986 in the names of Lynch et al.
A satisfactory solution for the problem of controlling, accurately and with rapid response time, the concentration level of the carrier gas and the vapor being delivered to a deposition site, such as a substrate tube used in the manufacture of optical preforms would be quite advantageous.